In the continuation of our studies of the structure of mitochondrial DNA, we are planning to generate HAEIII restriction endonuclease gel patterns of mitochondrial DNA from mouse L cell lines containing monomeric or dimeric mitochondrial DNA. The object of the work is to determine whether small deletions or additions have developed in the course of the formation of the dimers from the monomers. We are also studying the extent and the mechanism of penetration of ethidium bromide into purified SV40 virus. This reagent appears to cause a population of the virus to become unstable and to extrude DNA. We plan to continue our studies of the nature of the products formed when the nicking-closing enzyme acts on a single isomeric form of SV40 DNA having zero superhelical turns. The rate of formation of molecules with positive and negative superhelical turns will be followed as a function of time. The results of these experiments should illuminate outstanding questions relative to the mechanism of action of the nicking-closing enzyme. An investigation of the uses of a newly available small air-driven opaque ultracentrifuge will focus on elucidating the advantages and disadvantages of the equipment in buoyant density centrifugation experiments. BIBLIOGRAPHIC REFERENCES: The Action of Nicking-Closing Enzyme on Supercoiled and Nonsupercoiled Closed Circular DNA. Formation of a Boltzmann Distribution of Topological Isomers. David E. Pulleyblank, Mavis Shure, David Tang, Jerome Vinograd and Hans-Peter Vosberg. Proc. Nat. Acad. Sci. USA, 72 (11)4280-4284 (1975) Purification and Demonstration of the Enzymatic Character of the Nicking-Closing Protein from Mouse L. Cells. Hans-Peter Vosberg and Jerome Vinograd. Biochem. Biophys. Res. Commun., 68 (2) 456-464 (1976).